The long-term goal of this project is to understand the molecular mechanisms by which cell-cell signaling factors control vertebrate development, particularly development of the nervous system. The proposal focuses particularly on the ephrins, and their receptors the Eph tyrosine kinases. This large family of signaling molecules has unique roles in development, particularly in providing localized positional information that guides cells and axons. The main goal of this proposal is to characterize the molecular mechanisms of ephrin action, particularly signaling mechanisms that mediate axon guidance. One of the noble features of the ephrins is that these ligands are cell surface associated, and they signal through receptors by direct cell-cell contact. Much of this proposal focuses on the significance and mechanisms relating to the fact that these ligands act as cell surface molecules. In addition to elucidating ephrin biology, the results will have implications for signaling by direct cell-cell contact generally. Aim 1: In the case of the ephrins, it has already been shown that in addition to a signal through the receptor, a "reverse" signal can be transduced into the ligand-bearing cell. This aim is to characterize interactions of the ephrins with other molecules within the ligand-expressing cells, including the significance and mechanisms of reverse signaling. Aim 2: The expression of ephrins at the cell surface creatures an apparent paradox, since the receptor-ligand interaction would be expected to factor cell adhesion, yet can cause rapid cell detachment and repulsion. One solution could be studied proteolytic cleavage, and indeed a novel mechanism for regulated cleavage has now been identified and will be studied further. Aim 3: Forward signaling the Eph receptors is already known to have unique roles in providing detailed positional information for cell and axon guidance. Pathways of signaling will be characterized, with a particular emphasis on linking them to the ultimate effector mechanisms that change cell behavior. Regarding disease relevance, the work on axon guidance mechanisms could ultimately lead to new treatments to maintain or repair neural connections impaired by injuries, congenital abnormalities, or degenerative diseases. The work on mechanisms of migration, and regulated cell surface proteolysis, has specific implications in many areas, including immune function, cancer metastasis and Alzheimer's disease.